Crystallizable glass has many advantageous characteristics such as a thermal expansion coefficient close to that of silicon, a low dielectric constant, and a capability for firing at low temperatures. Therefore, high-density wiring substrates employing insulators that contain crystallizable glass in a major or minor amount and which are coated with conductive materials selected from among low-melting, low-resistance metals such as Ag and Cu have been widely used. A method has been proposed in Japanese Patent Application (OPI) No. 128899/1980 (term "OPI" hereinafter means unexamined Japanese Patent Application) in which a glassceramic structure is by first preparing an unfired ceramic sheet (hereinafter referred to as a green sheet) from glass particles that crystallize at a temperature lower than the melting point of copper, then printing a circuit pattern of a copper paste on the surface of the green sheet, laminating a plurality of thus-patterned green sheets into a unitary assembly, and firing the assembly in a pre-selected atmosphere such as a reducing atmosphere or an inert gas atmosphere.
The term "green sheet" as used herein means a "precursor" which forms a crystalline glass product when sintered; that is a material that can be fired at low temperatures and which possesses a comparatively low dielectric constant, a low thermal expansion coefficient and a great strength after firing. Such a green sheet may be prepared by the following procedures: a crystallizable glass component is prepared by adding 0.1-6 wt % of B.sub.2 O.sub.3 and/or P.sub.2 O.sub.5 to a mixture of main ingredients comprising 57-60 wt % of SiO.sub.2, 20-28 wt % of Al.sub.2 O.sub.3, 10-18 wt % of MgO and 2-6 wt % of ZnO; 100 parts by weight of the crystallizable glass component is ground into particles to form a first; 15 parts by weight of a butyral resin, 8 parts by weight of a plasticizer such as dibutyl phthalate, and 30 parts by weight of a solvent comprising a liquid mixture of acetone and toluene are added to the frit; the mixture is kneaded in a pot mill for about 48 hours to make a slurry; the slurry is then formed into a sheeting of a desired thickness by a suitable technique such as doctor blading.
The green sheet is also disclosed in Japanese Patent Application (OPI) Nos. 64545/84, 83957/84 and 137341/84.
In order to meet the demands of modern electronic devices and components featuring an ultra-high packaging density, higher performance characteristics and a lower production cost, it has been desired to develop a multi-layered substrate that is made of a copper-based conductive paste on a circuit substrate and in which the conductor area is more gas-tight than a conductor solely made of copper. It has also been desired that such a multi-layered substrate have an enhanced strength of adhesion to the copper paste after firing.
When a paste that is based on copper oxide is heated in a reducing atmosphere or an inert gas atmosphere, CuO is reduced to Cu which is electrically conductive. During this process, the paste undergoes a decrease or contraction in volume. On the other hand, a paste that is based on metallic copper undergoes an increase or expansion in volume as a result of oxidation of Cu during degreasing in the air atmosphere. Such a contraction or expansion in volume can cause cracking in the ceramic portion formed by firing the green sheet or in the conductor area.
If holes extending through ("through-holes") the ceramic portion are filled with a conductor paste, not only does the contraction described above occur, but also a thermal contraction mismatch that occurs between Cu and the ceramic portion during firing can produce either a gap between the inner surface of such holes and the conductor or cracking in the conductor area, thereby leading to poor electrical conduction.